Retrotransposable elements (RTEs) comprise ~45% of the human genome. They are mobile DNA elements that can insert into new genomic sites using a copy and paste process. All organisms have evolved multiple silencing mechanisms to protect themselves against RTEs. These defenses are quite effective, and hence the great majority of RTEs have become passive genome passengers, accumulating mutations over evolutionary time. Most organisms, however, harbor a small number of elements that remain active; in humans, the long interspersed nuclear element-1 (LINE-1, or L1 for short). Currently most of what is known about RTE activity stems from studies of germline activity or from experimental models in which RTE activity is driven by artificial constructs. At the normal somatic cell level, the magnitude of the impact of RTE activity is poorly understood. However, a rapidly increasing literature shows that somatic retrotransposition is much more frequent than previously believed. The team of this Program Project Grant (PPG) proposal has assembled compelling evidence that somatic retrotransposition is activated during aging, and that this process is evolutionarily conserved. The PPG is founded on the hypothesis that the somatic activation of retrotransposition is a novel and hitherto unappreciated molecular aging process, and the goal of the PPG is to shed light on this new and fascinating aspect of RTE biology. During the first two years of research funded by this PPG we discovered that: 1) A variety of RTEs are induced with age in Drosophila, and antagonizing their activity by several methods extends normal lifespan; 2) L1 elements are strongly activated in human cells during all forms of cellular senescence; 3) L1 elements are activated with age in multiple tissues in the mouse (including the brain); 4) The activation of L1 elements in senescent human cells as well as normal mouse tissues triggers a strong Type I Interferon (IFN-I) response that promotes inflammation; 5) Treatment of cells as well as mice with nucleoside reverse transcriptase inhibitors (NRTIs) alleviates IFN-I activation and its proinflammatory effects. In this Supplement, Project 1 (Regulation of Retrotransposable Element Activity in Cellular Senescence and Aging, Sedivy) and Core B (Transposon Engineering and Genomics, Boeke) propose to extend this research by directly examining these processes in the context of Alzheimer's disease (AD). Specifically, the occurrence of novel L1 insertion events in the cortex of AD patients will be examined using a new and highly sensitive retrotransposition assay (TIP-seq) developed by Core B. This direction is founded on the hypothesis that the activation of L1 elements with age in some cells of the brain contributes to the development of AD or exacerbates the phenotypes of AD. The research proposed in this Supplement, enabled by our recent findings and methodological advances, is responsive to the scope of the Overall Specific Aims of the Parent Grant, in particular Aim 1, to define the landscape of somatic retrotransposition in aged tissues and senescent cells, and Aim 3, to study the downstream consequences of RTE activation on cellular and tissue function.